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Document transcript

Project summaries.

ALICE Management Committee meeting :27 July

(8 August)2012.

Machine development

Photoelectron Gun was successfully HV conditioned to >400kV after cathode change andgun bakeout. Bare wafer had no field emission at the test voltage of 370kV. The first cathodeactivation achieved just under 3% QE.

Cathode lifetime is reasonable : re-Caesiation is normally required once a week. But the gun

vacuum is low 10-11mbar (an order of magnitude higher than we used to have) due to a failureof one of the NEGs.

First use of INJ quads degausser had a huge effect on beam optics in ALICE injector thusnecessitating re-tuning all setups.Work on varioussetups continued.

New AP setup at 60pC have been largely established that could be used for both transverseand longitudinal beam dynamics experiments.

Towards the end of July, FEL stopped lasing

for currently (as of 27/07/2012) unknownreasons;investigation continues.

Several machine related issues have been investigated: stability (while there was a period oflarge beam instability in July); beam loading effects.

New EMMA style electronics was tested on ALICE and proved to be a very useful tool.

AP: transverse dynamics

Transverse dynamics studies on ALICE have concentrated on further analysis work usingquadrupole scans around the machine. Work was performed in both the injector and post-linac sections of the machine to characterise the transverse dynamics, and to attempt tounderstand the results previously seen.

The work is ongoing, but has demonstrated the ability to characterise and correct transverseoptics in the post-linac section. This is especially important in the AEMITER working mode,where the low bunch charge of ~20pC enables a more Guassian beam shape, improving theanalysis, but which has much stricter requirements on beam optics in the interaction anddetection regions.

Transverse dynamics in the injector area have also been analysed in terms of emittance, andshow the expected behaviour versus buncher power. These studies have demonstrated thebenefits of the new 325keV working point.

A major component of the ongoing work is to understand the effects of bunch charge ontransverse dynamics, and this has been investigated thoroughly in terms of the 2-beamdynamics studies. This novel analysis has investigated the high bunch charge, strongbunching, effects that lead to a double-beam, with different transverse properties, throughoutthe machine.

Transverse optics set-ups for the various working modes of ALICE, have been successfulthroughout the current period. Standard set-ups for AEMITER, FELIS, AP and THz modeshave all been developed, and show relatively consistent behaviour over the period. Limitingfactors for the transverse dynamics repeatability are related to the remnant fields in themagnets, as well as large variability in the longitudinal dynamics which necessitate newtransverse dynamics modifications.

AP:longitudinal

dynamics

Bunch evolution in injector. Some linac-exit bunch length measurements were taken in the‘set-up’ period in the final week of June, using RF streak methods. This confirmed theobservations earlier in the year that the booster chirp seemed to have only small effect on thelinac-exit bunch length for the current injector set-up. This is somewhat unexpected andfurther data/simulations are required to understand the effect.

Path length Measurements. Time of flight measurements have continued using highresolution oscilloscope and BPM signals. These have demonstrated the ability to measureTOF differences to a couple of pico seconds and thus can measure in principle

R56 values of~ cm level. The effect of lattice tuning including arc quadrupole and sextuple strength on thetime of flight has been measured, but the repeatability of the results and comparison withtheoretical expectation is not clear and work is underway to understand the experimentalartefacts involved in the method. Elegant simulations have shows some agreement

with thedata on the shape of the TOF vs energy curve and the influence of arc sextupole.

Single Bunch BPMs. Primarily to aid the understanding of the path length measurements,single bunch BPM electronics have been used on the arc BPMs. These have providedinformation on the energy of the bunches over the train and the alignment of the magnets inthe arc. In particular they are useful to understand the offset of the beam in the sextupoleswhich is crucial to longitudinal dynamics.

2 scheduled AP shifts were lost to RF faults on Sun 1st

of July.

FEL

FEL lasing was restored very early in the running period after the cathode change (in the firstfull shift). A reasonable setup was established with similar power levels to previousrunningperiods, however the performance in terms of stability has been mixed–

a few shifts withhigh stability but more often not so good. This has been a cross-machine problem and theFEL has been used to investigate the source of the instability, in addition to working toimprove the tolerance of the FEL setup to instability. Further work is required on improvingthe setup–

particularly since some recent changes have made restoring lasing problematic.

A fast-response detector was installed in the accelerator hall and has been used to resolveindividual pulses in the FEL output. Data was recorded to evaluate the gain of the FEL, withthe intention for this detector to be used in future as an online diagnostic of FELperformance. A laser used for monitoring the FEL cavity mirror angles had failed and hasbeen replaced.

A new aim for the project is to demonstrate lasing at 20um (a particularly useful wavelengthfor scientific studies using the output), which requires a

new electron beam setup at lowerenergy. A few attempts have been scheduled but not completed due to various difficulties, sothis remains a priority for future shifts. A detector sensitive to this wavelength has beensourced and installed.

One other significant development is that several

non-FEL-specialist members of thecommissioning team have successfully established lasing in this period. This increases thepotential for utilising the FEL in conjunction with other accelerator physics projects, plusproviding additional support for FEL-user projects such as FELIS.

EMMA

"EMMA had beam time for three days from 16 to 18 July. A beam in EMMA seemed verygood or even better than ever before probably because of careful tuning by JJ and/or morestable operation of ALICE. We spent shifts for two subjects. One is the investigation ofEPICS/BPM system and the other is the COD analysis. Unfortunately, the EPICS/BPMsystem is still not fully functioning. In order to pin down where the faults occurs, GC andRichard Burrell who is a designer of the digital system from a company spent large fractionof time. It suggested some improvement scenario to be tested later. Regarding the CODanalysis, DK, IK and JG took a response function by moving

a couple of magnetstransversely. Hopefully, the detailed analysis of this pilot measurement will fix theparameters to be used for the response function measurement of the entire magnets."

AEMITR

Last year, we reached the stage of running some full system tests (including THz generationin the accelerator area, achieving the required electron beam optics, and operating all thediagnostics).

The results we obtained showed (as we suspected) a level of beam size jitterthat could make it difficult to identify the signal we are looking for (a variation in beam sizein the chicane, indicating an increased energy spread resulting from interaction with the THzpulse).

Although we could in principle average over many pulses to try to improve theeffective signal-to-noise ratio, the problem is that our previous timing technique only enabledus to synchronise the electron and THz pulses to within (roughly) a nanosecond, whereas theelectron beam is a couple of picoseconds, and the THz pulse perhaps less than that.

Thismeans we need to make long timing scans while lookingfor a signal, which already takes aconsiderable time, even without averaging.

To try to improve our sensitivity to what we expect to be a rather small signal, we areworking on two significant modifications to the set-up:

1.

Using fast gated cameras in

the chicane, we should be able to record YAG images fromtwo bunches within the same train.

We expect that the variation in beam size betweenbunches in the same train should be significantly smaller than the variation in bunches fromdifferent trains; therefore, by looking for differences between bunches within the same trainwe should achieve much better sensitivity to the THz interaction.

2.

Use of the streak camera to synchronise the THz and electron bunches to within a fewpicoseconds.

This will enable us to concentrate our search in regions where we have a muchbetter confidence that the interaction should be.

In the shifts we have had so far this year, the operation of the gated cameras for observingdifferent bunches within a single train has been demonstrated.

Initial attempts to use thestreak camera identified some optics issues (to do with directing and focusing the image fromthe OTR screen onto the camera), which should be resolved with some further work.

At thesame time, continued experiments with THz production in the Diagnostics Room have givena better understanding of the properties of the THz beam, including the focusing required toproduce an observable effect on the electron beam.

The goal is to demonstrate an energy modulationon the electron beam by interaction with theTHz pulse, by the end of this year.

FELIS

First snom beamtime in July allowed re-installing the microscope and testing improved dataacquisition systems which include incident flux normalisation. Some beam instability in thefirst part of the beamtime gave opportunity for this to be tested and the expectedimprovement in snom images were obtained by normalising to the incident flux. A shortamount of time was used to try to improve the coupling of light from the

fibre onto the MCTdetector element but this will now be perused off line using a broadband thermal source. TheSNOM tip was used to collect the reflected IR light as a function of wavelength for standardsamples and for one tissue sample at various points on each of the samples. The localabsorption is a small component of the total snom signal and a better understanding of basicphysics of the snom process will now be a major component of a more focussed EPSRCproposal by PW.

THz

High levels of THz

were generated and transmitted to the diagnostics room during start up.Although beamline transmission dropped for the exploitation slot on 15 July, Martin Volkwas still able to obtain data from peptides in solution as a function of temperature, using a

custom cell with TPX windows. Another microfluidic cell equipped with silicon windowswas tested but found to cut out much more light. This will now be modify and theexperiment re-run on 28 July. A coupler on the THz M1 mirror drive has failed, preventinguse of the AP THz detector. This has now been repaired and the beamline will be re-optimised on 28 July prior to THz experiments.

THz CTR

A small nickel coated silicon screen is in place in the machine and a small breadboard for testdetectors in place for initial tests of THz generation by CTR. A larger screen and appropriateholder are now available for future installation.